1. Field of the Invention
This invention relates in general to radar systems, and in particular, to a unit for use with magnetron radar transmitters which will correct receiver signals for transmitter output errors.
2. Description of the Prior Art
Radar systems in general send radio frequency waves out from a transmitter. If these waves contact an object, the waves will be reflected back and may be received by a receiver. The reflected signal can be analyzed to determine the size, range and velocity of the object. The amplitude of the waveform received indicates the size of the target or object. The time delay indicates the range or distance to the target. The change in frequency indicates the velocity of the target.
Generally, the radio frequency signals are high frequency energy bursts or pulses; for example three gigahertz (3.times.10.sup.9 cycles per second) for a pulse period, followed by an off duty period, then followed by another pulse of the same length, frequency and duration. If the receiver antenna is pointed high into the air, where any type of reflected wave would indicate a flying object, then the reflected waveform can be analyzed to determine the nature of the target. However, low flying aircraft could fly below the radar antenna pattern if the receiver is pointed high into the air, and the receiver would not be able to detect the presence of the aircraft. Pointing the receiver at a low angle so as to avoid missing low flying aircraft creates other problems. The receiver will receive reflected signals possibly due to trees, hills, mountains, buildings and other non-flying objects. The reflected signal due to these objects is called clutter. Clutter is often orders of magnitude larger than the signals of interest, and imparts no information to the system of value, and obscures the signals which do bear information.
Systems are available to determine what part of the reflected signal is due to clutter and what part is due to a flying object, if any. The pulse envelopes being received are compared. If sequential received pulses are substantially identical in frequency, then the reflections are all due to stationary objects. Moving objects will cause a variance between pulse frequency. In the past, to be able to compare these pulses and determine which is clutter, the system must have a very stable transmitter. The transmitter must produce substantially identical pulses in phase, frequency and amplitude. High power systems that do this normally employ a klystron transmitter. The processing which separates the clutter is known as phase dependent processing.
The klystron systems work well, but they are expensive, bulky and inefficient in power compared to older systems which employ magnetron transmitters. The magnetron is a tube system employing very high energy to generate the radio frequency signals. The trouble with the magnetron is that the pulse envelopes transmitted vary from one another in frequency, amplitude and phase. Consequently, a phase dependent processing unit cannot separate out the clutter very well. The pulses being received by the receiver, even if reflected from a stationary object, will not be sutstantially identical because the transmitted pulses transmitted by the magnetron are not identical. Some magnetron systems employ features to improve the stability of the transmitted signal. For example, one technique known as COHO injection employs a stable transmitter as a primer for the magnetron to initiate a more stable output. Another prior art technique, known as COHO locked, is one in which some of the instabilities of the transmitter are impressed on the otherwise stable receiver to effect cancellation of those instabilities on receive. However, the rejected clutter performance even with these systems is far worse than the klystron systems.